A normally on type field effect transistor (FET) is inverted into the OFF state by applying a constant voltage to the gate thereof. For example, most FETs including GaN semiconductors in the active region thereof, such as GaN FETs (are normally on type n-channel FETs. Therefore, in order to invert the GaN FET into the OFF state, it is necessary to apply a negative voltage of not more than a certain level. However, if the gate voltage rises due to gate leakage, it will be difficult to maintain the OFF state.